In the art of semiconductor manufacture it is known that the minimum thickness of a plane semiconductor body into which a semiconductor device is to be incorporated is restricted, in general, by the requirement for the body to be sufficiently rigid and robust so that it can be handled without breaking. This can mean that a device has to be made thicker than is necessary from a theoretical viewpoint. This is disadvantageous, firstly because more semiconductor material is used than is in fact required so that the device is more expensive than it need be. Secondly, some of the device characteristics may be adversely influenced. In practice, therefore, a compromise is reached between a device which is sufficiently thin for it to have acceptable device characteristics and a device which is sufficiently thick so that it can be handled without damage.
In order to illustrate these problems more clearly one particular semiconductor device, namely a semiconductor rectifier, will now be considered in more detail.
It is known that the reverse voltage at which a semiconductor rectifier breaks down can be increased by including in the semiconductor body of the rectifier a high resistivity intrinsic region between two semiconductor regions of opposite conductivity type. A semiconductor rectifier having this structure is known as a p-i-n diode. The voltage at which the device breaks down, the breakdown voltage as it is called, increases with increased width of the intrinsic region, which width is referred to hereinafter as the base width. However, the base width also adversely affects the forward current-voltage characteristic of the rectifier and so it is preferable that the base is not, or is not much wider than that necessary to obtain a device with the desired breakdown voltage. These base widths can be so small that it is difficult to make a device with optimum design. This is because regions of the body on each side of the base are of the n- and p-conductivity types respectively. Typically these regions are formed by diffusion. However, it is not possible to carry out deep diffusions accurately. In fact the deeper the diffusion the less well defined will be the positions of the junctions between the different regions and the consequence of poorly defined junction position is a device with poor and unpredictable characteristics. For a device which is to operate up to a given breakdown voltage and for which the optimum thickness of the body is less than that which is realizable in practice a compromise is made as follows. The intrinsic region is made sufficiently thin for the junctions between the different regions to be reasonably well defined in order to obtain low forward voltage drops while this same region is sufficiently thick to prevent breakdown at the desired operating voltages.
To avoid the difficulty of deep diffusions p-i-n diodes with relatively narrow base widths can be made using epitaxy to grow the intrinsic region on a relatively thick substrate of a first conductivity type. An epitaxial region of the opposite conductivity type is then grown on the intrinsic region. A p-i-n diode made using epitaxy is described in the Japanese Patent Application published under No. 55-18010. However, the device described there also includes a so-called `wave-shaped` p-n junction which is said to reduce the forward voltage drop of the diode because of the decreased current density resulting from the increased junction area. As the diode is formed using epitaxy one of the major surfaces of the semiconductor body also has a wave shape, the depth of the waves being several micrometers. Despite this, the decreased current density is not a feature of the device as a whole because the opposite major surface of the completed diode is planar. Therefore, the current handling capability of the device is not altered by the wave-shaped junction and, moreover, the base width of the diode is not constant. Also, epitaxy is a relatively expensive technique and suffers from the drawback that, if there are any defects in the substrate, they can propagate through the structure as the epitaxial layers are grown. Furthermore, because the diode comprises a relatively thick substrate, it again includes more semiconductor material - and so is more expensive - than is necessary from the theoretical standpoint.
There is another problem associated with semiconductor rectifiers which either have a wave-structure as described in the above-mentioned Japanese Patent Application or which are formed from plane semiconductor bodies, namely premature breakdown. Because of surface field effects such devices tend to break down at the periphery before they do so at the center. One attampt to solve this problem is described in U.S. Pat. No. 3,428,870. The solution relies on forming a lower resistivity peripheral portion of the intrinsic region. However this provides no solution to solving the problem related to narrow base widths.
Another solution to the problem of premature breakdown is described in U.K. patent specification No. 1,105,177. In this case the plane semiconductor body is etched from one major surface to form a thinner central portion which is substantially flat and which is surrounded by a thicker peripheral portion. The region adjoining the etched surface is diffused in after etching so that it is of uniform thickness throughout the body. The opposite conductivity type region adjoining the other surface is also of uniform thickness. Consequently the intrinsic region comprises a thin central portion surrounded by a thicker peripheral portion. The voltage at which the peripheral portion breaks down can thus be increased so that it does not break down before the central portion. Clearly this solution also goes some way to solving the problem of narrow base widths, the thicker peripheral portion gives a degree of rigidity to the thinned device. However, particularly in the case of large area devices for handling large currents across the device, the minimum thickness is still restricted by the requirement for mechanical strength and rigidity so that the compromise mentioned above must still be made to some extent.